How does null-balance potentiometer indicate the temperature of a thermocouple?

A null-balance potentiometer indicates the temperature of a thermocouple by,,, A. measuring the current with a galvanometer. B. measuring the voltage across the thermocouple reference junction. C. making the slide-wire voltage equal to the voltage from the thermocouple. After reading the book for quite some time i would have to say that C would be the correct answer, Any others agree or disagree ?